EGG A fully formed and newly egg is large and elliptical with one endbroader than the other .TYPE The egg of chick is telolecithal ,sub type of polylecithal.RELEASE The egg is released from the ovary, it takes 24 hours to passdown the oviduct, before being laid .FERTILIZATION The fertilization is internal ,upper part of the oviduct, resultingtwo polar bodies degenerate and disappear.
STRUCTURE OF EGGSHAPEIt is oval in shapeSIZE3cm wide and 5cm longSHELLMade up 95% calcium carbonate, white and porous and exchange of gases occur, softand flexible in a freshly –laid egg but soon become hard and brittle.MEMBRANETwo types of membrane, shell membrane and vitalline membrane , separated at thebroad end of the egg to enclose an air space
YOLK MEMBRANEThe layer of yellow yolk are thicker than those of the white yolks. Yolk consist ofphospholipids, lecithinSolid yolk shows 60% fats and 70% proteinsWhit yolk contains less fat and also less of fat soluble carotene to which the yellowcolour is dueYolk contains about 50% waterALBUMENIt has two types Thick albumen and thin albumenCHALAZATwo end of the egg are some fibrous ,rope like twisted structure formed duringjourney of the oviductLATEBRAThe central flask shaped area that is enclosed of white yolk is called latebraGERMINAL DISC OR BLASTODICNucleus of the egg is surrounded by the negligible amount of yolk free cytoplasmcalled germinal disc
DEVELOPMENT OF CHICKCLEAVAGE OR SEGMENTATIONIt starts with the cleavageIt is meroblastic means confined to small area of blastodiscDuration it takes 12 16 hoursFirst cleavage forms two cellsSecond cleavage occurs after 20 min of first cleavage four cells formedThird cleavage eight cells formedIrregular cleavageoccurs in four hours at the end cleavage 256 irregular cells formedBLASTULATIONAfter cleavage, blastulation occurs, due to blastulation following structure formedSub-germinal cavity : it is similar to blastocoelArea pellucida: Form the body properArea opaca: Help in the formation of extra embryonic membrane such as yolk sac
PROCESS OF GASTRULATIONConverting of monoblastic egg into triploloblastic gastrulaGastrulation involves the formation of ectoderm, endoderm andmesodermPRIMITIVE STREAK FORMATIONThe primitive streak is formed from the posterior marginal regioncells .The streak first becomes visible as the epiblast (ectoderm) thickensat the posterior pole .PRIMITIVE STREAK FORMATION 7-8 HOURS AFTERFERTILIZATIONThe epiblast (ectoderm) is separated from the hypoblast by theblastocoelThe epiblast will give rise to the three germ layers the embryo, whilethe hypoblast will give rise to extra embryonic structure.PRIMITIVE STREAK EXTENTION 15-16 HOURS AFTERFERTILIZATIONThe primitive streak extends by convergent extention from theposterior to anterior pole. A depression form along the primitivestreak called primitive groove.
FORMATION OF HENSON’S NODE 19-22 HOURS AFTERFERTILIZATIONThis bird eyes view shows the primitive streak and Henson’s node .At this stage ingression through the primitive streak of cells destined tobecome endoderm and mesoderm has begun.HENSON’S NODEIt is formed by cells of presumptive notochord and floor of neural tube .FORMATION OF NOTOCHORD (NOTOGENESIS)A narrow strip of blastoderm just in front of primitive knot consist ofpresumptive notochordal cell responsible for the formation of notochord.FORMATION OF NEURAL TUBE (NEUROGENESIS)In front of the primitive streak lie neural plate cells . These cells folds roll upand unite mid-dorsally ,enclosing a neural tube , forming fore brain , midand hind brain.MEROBLASTIC SOMITESOn either side of notochord lies thick, solid, dorsal somatic
ORIGIN OF COELOMThe ventral or lateral plate mesoderm split into two layersThe outer somatic or parietal layer lies next to ectodermwith which it forms the somatopleureThe inner splanchnic or visceral layer in contact withendoderm form the splanchnoplure.Space or cavity formed between these two mesodermallayers is coelom or splanchnocoel.ORGANOGENYFormation of organs is called organogeny
EXTRA EMBRYONIC MEMBRANE OR FOETAL MEMBRANEYOLK SACThe chief embryonic food is yolk which is surrounded by the sac likeinvesting membrane called yolk sacYolk sac is made up of splanchopluereFUNCTION:The function of yolk sac is to protect the yolk keep it in position, digest and absorb it.Yolk sac serves as a primary organ of nutrition of embryoAMNION :It is made up of inner ectoderm, the space between amnion andembryo is filled with amniotic cavity having amniotic fluid.FUNCTIONIt protect the embryo from mechanical jerks and prevent itsdesication.
CHORIONIt is made up of outer ectoderm ,the cavity between amnionand chorion is called chorionic cavityFUNCTIONChorion also provides the liquid medium for the embryo.ALLANTOISEThe bladder like structure is called allantoiseAllontoise is made up of splanchnopleure endoderm insideand splanchnic mesoderm outsideALLANTO- CHORIONThe mesodermal layers joined the allantoise with chorioncalled allanto-chorion , a compound layer is formedFUNCTIONIt serves as Respiratory organ ,Excretory organ, Nutritionalorgan